Sugar polymers or exopolysaccharides (EPS) produced by lactic acid bacteria (LAB) can be subdivided into two groups: homopolysaccharides and heteropolysaccharides (Cerning, 1990, 1995; De Vuyst & Degeest, 1999). Four groups of homopolysaccharides can be distinguished: α-D-glucans, β-D-glucans, β-D-fructans, and others like polygalactan (Monsan et al., 2001). Strain-specific differences occur that depend on the degree of branching and the different linking sides. Heteropolysaccharides are produced by LAB in a greater variety concerning chemical composition, monomer ratio, molecular mass and molecular structure (De Vuyst et al., 2001; Faber et al., 2002). Their repeating units most often contain a combination of D-glucose, D-galactose, and L-rhamnose, and, in a few cases, fucose, nononic acid, ribose, acetylated amino sugars and glucuronic acid, as well as non-carbohydrate substituents such as phosphate, acetyl and glycerol. Heteropolysaccharides receive renewed interest because of their functional role in food systems such as enhancement of viscosity, suspension of particulates, inhibition of syneresis, stabilization, and emulsification. For instance, the use of ropy, EPS-producing LAB strains is applied in the manufacture of yoghurt to obtain a smooth texture and a good mouthfeel.
The in situ production of these sugar polymers by the yoghurt bacteria is applied as an alternative to the addition of animal hydrocolloids (gelatine and casein) or chemically modified plant carbohydrates (starch, pectin, guar gum, etc.). EPS from non-GRAS (Generally Recognized As Safe) microbial origin (xanthan or gellan) are used in foods as well. However, not all these food additives are allowed in all countries.
Furthermore, the amounts of EPS produced by LAB are low and their production is unstable, particularly in milk (Degeest et al., 2001b).
Due to the low amount of EPS produced by LAB strains and the transitory nature of the exopolysaccharide trait (Cerning, 1990), the use of these compounds as food-grade additives is still limited. Until present, only dextran homopolysaccharides from LAB found industrial applications, albeit in the non-food sector (Monsan et al., 2001).
Further, the construction of genetically modified strains might allow the production of higher levels of EPS or new biopolymers. However, the use of genetically modified microorganisms in the food industry is hindered by the hostility of the consumer. Therefore there is a growing need for the optimalisation of fermentations with respect to physical and environmental factors, with rationally selected strains for use in the food industry.
There is further an industrial need for strains that produce high amounts of stable EPS in a safe and simple environment.